This invention is concerned with a hereditary pain disorder known as “Congenital Indifference To Pain” (also referred to herein as “C-I-P”). It is an extremely rare but interesting medical condition. It is an autosomal recessive disorder that interferes with the normal perception of pain (see, Landrieu, P. S. G., and Allaire, C. Ann. Neurology, 27 (5):574-58 (1990) and Comings D E and Amromin G D, 1974. Neurology. September; 24(9):838-48.)
The description of the condition is fascinating. Patients are essentially completely indifferent to sensations that would cause pain in most individuals; yet at the same time they are quite able to distinguish between other sensations, such as thermal (hot/cold) and tactile (sharp/dull) sensations. Tendon reflex and vibration recognition is normal. Patients do not perceive inflammatory pain or dental pain even though there is a normal flare response to intradermal histidine injection, indicating that the peripheral arc reflex is intact. (Landrieu et al, 1990) A general description of C-I-P can be found at Online Mendelian Inheritance in Man (OMIM) reference *243000.
It is highly intriguing that the genetic mutation that underlies this disorder has such a specific and selective medical consequence. The inventors recognize that identification of the mutant gene which is associated with the disorder will provide a therapeutic target against which novel therapeutic agents can act. The inventors recognize that therapeutic agents which are highly selective for this therapeutic target, if they can be designed to mimic the effect of the mutation, have the potential to induce the same kind of analgesia, in the general population, as experienced by C-I-P patients.
It thus serves to examine in detail the physiological consequences of C-I-P. One C-I-P patient (3 years old, female) had no pain sensation but had normal thermal and tactile sensation and deep sensation. She appeared normal physically, although having tissue damage presumably resulting from the disorder, such as a shorter tongue, lack of some fingertips and multiple scars on fingers due to old burns. She learned to walk at 18 months (somewhat delayed). (Guillermo A. and A. Grinspan, “Congenital indifference to pain, a propos of a case with antecedents of consanguinity”. Rev Neurol (Paris), 1970. 123(6): 434-5). In a follow up examination at age 34, patient appeared normal. Intelligence was normal. A broken bone not identified by the patient had resulted in an orthopedic deformity. Patient had a dental prosthesis. Sweating was normal. Tendon reflex and vibration recognition was normal. Position sense was normal. Patient was able to discriminate between “hot and cold” and between “sharp and dull” but discrimination between “touch” and “pinprick” was attenuated. Her maternal grandfather and her paternal grandmother were first cousins who had a common cousin who was apparently indifferent to pain. This consanguinity is in support of autosomal recessive inheritance.
Sweating, blood pressure and other autonomic responses are quite normal. No histological abnormalities are present in the peripheral nerves. (see Comings et al (1974) and Hirsch E, Moye D, Dimon J H 3rd. South Med J, 88(8):851-857 (1995)).
Still, to date, no genes or genetic loci responsible for C-I-P have been reported. Karyotype analysis of one C-I-P patient appeared normal. (Amguerra-Escobio, 2001, unpublished observations). One of the confusing aspects that has prevented identification of genes in the past is that there are other patients who have other hereditary pain disorders which have not been properly distinguished from autosomal recessive C-I-P. This makes it significantly harder to identify the common underlying genetic cause of the disorder.
For example, there are autosomal dominant and sporadic cases of congenital indifference to pain. The inventors believe that many of these cases have a different genetic basis than autosomal recessive C-I-P. Reports of these cases may be found at Becak et al, Acta Genet. Statist. Med. 14:133-142 (1964); Comings et al (1974); and in Landrieu et al, (1990).
Further, there is a very different from of the disease known as Hereditary Sensory and Autonomic Neuropathy (HSAN). HSAN is the classic “insensitivity” to pain. It has been classified into types I-V (Reilly, M. M. 1998. J. Neurol. 245(1):6-13). Each of these conditions is linked to an inherited neuropathy of sensory nerves. Some of these conditions have been linked to genes or genetic loci, however, none of these are believed to correspond to the genetic basis underlying C-I-P.
It should be recognized that “insensitivity” and “indifference” are often used interchangeably in the art, but they should be properly distinguished for better understanding of the pathology of the disorder. The term “insensitivity” refers to situations in which sensory pathways are altered leading to lack of painful sensation while “indifference” refers to situations wherein the sensory pathway appears normal but there is a lack of reaction to a painful stimulus. The key distinction between these groupings is that the former are inherited neuropathies in which neuropathy is the primary part of the disease and the sensory pathway is the sole or primary part of the neuropathy, whereas in the latter, the sensory pathway appears, at a microscopic and macroscopic level, to be intact and normal. (Comings et al 1974)
Because this invention is directed towards genes, proteins and other tools which are targets for analgesic agents, an additional area of relevant background relates to existing analgesic agents and their therapeutic targets, where known.
Therapeutic agents for treatment of pain fall into two main classes—the NSAIDs (non-steroidal anti-inflammatory drugs) and the opioids. NSAIDs treat pain in a way similar to the mechanism of aspirin, the most well-known and oldest member of the class. Common NSAIDs include acetaminophen, ibuprofen and naproxen. These drugs mainly inhibit the body's ability to synthesize prostaglandins. The common mechanism of action for all NSAIDs is the inhibition of the enzyme cyclooxygenase (COX). A major commercial success has been achieved with specific inhibitors of COX-2, such as Celebrex™ from Pharmacia/Pfizer, and Vioxx™ from Merck & Co. The recently launched Bextra™ from Pharmacia/Pfizer, which is highly selective for inhibition of COX-2 over COX-1, is also expected to become commercial success.
Opioids act through the opioid receptor family. These drugs include the weak opioids such as codeine and Tylenol 3, and strong opioids such as morphine and methadone. Some are long acting, others are of short duration. Opioid analgesics have a tendency to addiction and dependency, and so are not ideal for long-term or chronic pain management.
Outside of the NSAIDs and opiods, there are a number of other suggested analgesic agents in clinical trials (i.e. not yet approved for marketing) which are believed to have alternative targets. Some clinical trials are attempting to establish that central neuropathic pain may respond to ion channel blockers such as blockers of calcium, sodium and/or NMDA (N-methyl-D-aspartate) channels. For example, in development are low affinity NMDA channel blocking agents for the treatment of neuropathic pain. The literature provides substantial pre-clinical electrophysiological evidence in support of the use of NMDA antagonists in the treatment of neuropathic pain. Such agents also may find use in the control of pain after tolerance to opioid analgesia occurs, particularly in cancer patients.
In accordance with the present invention, the identification of the hereditary basis for Congenital Indifference To Pain will be a key step for developing novel therapeutic agents because it has the potential to be a novel therapeutic target. This therapeutic target can be used to identify and discover more effective analgesics. Discovery of the target will also provide new methods and compositions for diagnosis of C-I-P and for distinguishing between types of inherited pain disorders.